rain, rain, go away

Yes, it sometimes rains in desert. I got about 20 drops last night, and a few hundred drops this afternoon. It’s supposed to be cloudy with rain today and tomorrow, then back to full sun Monday and thereafter.

This is a great time remind folks of the…

limitations of solar power

I think this is something newbies need to hear, particularly those thinking about running their entire rig of 100-200w of panel alone.

It is medium-dark overcast and 39F in Pahrump, NV right now with light rain. It was a lighter about an hour before when I took this pic of wild horses grazing nearby:


Here is my actual harvest from 570w of panel on an MPPT controller:

  • 3:37pm -35.61v @ 0.64A = 23.01w, or 4% of rated output. Loads 37w, net 17w drain

At the $1/watt people typically pay for small solar setups that would be $33.46/w produced in this scenario. {At my actual cost for these bigger NOS panels it is $24.34/w produced, still very expensive}

There’s not a good way to boost this solar output. We are already running MPPT. Adding more panel wouldn’t make a big difference here in absolute terms - we could increase to 1000w of panel and be producing 40w, yay!. Tilting the panel makes no discernible difference in overcast conditions. Even if it had maximal effect (+30%) we’d be up to 30.5w; yay again!

comparison to isolator (alternator charging)

The battery bank was holding on by its fingernails to 12.9v, but I decided to crank up the engine to see **what kind of power the isolator would provide. **

  • immediately after cranking the monitor showed Vbatt rising rapidly through the thirteens at ~295w. Based on the cost of the isolator install this would be $0.27/w.
  • within 30 seconds the batteries had come up to 14v (alternator voltage) and current was dropping; you can think of this as a pseudo-absorption. 219w and falling. $0.37/w.
  • when the bank is fully cycled to 50% DoD the isolator initially provides about 475w, or $0.17/w

the takeaway

Solar under crappy skies is expensive and makes very little power. We either have to drastically reduce consumption or augment with another source of power.

Considering only the costs of the gear, the isolator makes well over 100x the amount of power per dollar than solar under crappy skies. Of course we should think about wear/tear on the vehicle and fuel consumption, but the difference is drastic.

Under good sun the cost of the solar harvest would be something like $1.81/w ** produced for typical installs and **$1.32/w produced for the less expensive bigger panels. A much better cost/benefit once you factor in the wear/tear of alternator charging.